1. Field of the Invention
This invention relates generally to photocopying, and is particularly concerned with image development in electrostatic photocopying equipment of the type which employs a liquid toner.
2. Description of the Prior Art
The liquid developers used in electrophotography comprise a dispersion of solid toner particles in a liquid having dielectric properties. During development, the toner particles, which bear a charge, are attracted to the electrophotographic material in accordance with the charge of the image upon it, whereas the dielectric liquid flows off. The result is a visible image.
Normally the electrophotographic material is brought into contact with the liquid developer by drawing it through a container which contains the liquid developer (a "bath"), or a thin film of liquid developer is applied to the surface of the material by spraying or by transfer roll.
An improvement in the bath type of development apparatus is described in U.S. Pat. No. 3,605,693 granted Sept. 20, 1971. This patent discloses a perforate, electrically floating electrode employed in a liquid toner bath for electrostatic image development. Such an electrode, placed in close proximity to the image-bearing surface of the electrophotographic material during development, is referred to in the art as a "development" or "developing" electrode. The essential characteristics of a developing electrode are that is is made of an electrically conductive material and is maintained in close proximity or virtual contact with the image side of the electrophotographic material. The development electrode has the advantage of overcoming the difficulty commonly known as "edge effect", the tendency of the copying process to wash out the interior of an extended black area of the image, coloring only the edges of that area black. This is especially important when the electrophotographic material is intended for use as a lithographic printing plate, because, if the image formed on the printing plate exhibits this edge effect, the defect will be transferred to all copies made from the plate.
While the development electrode described in this U.S. Pat. No. 3,605,693 may be capable of producing high quality electrostatic reproductions, the bath type of developer suffers from several disadvantages, particularly when the imaged material is paper. When the sheet is immersed in the toner bath, it absorbs or picks up a considerable amount of carrier liquid for the toner particles and carries the liquid with it from the bath. This amount of liquid must then be evaporated from the sheet. This evaporation requires energy and time for drying the sheet, and replenishment of the bath. Furthermore, maintenance of the "virtual contact" between the paper and the stationary electrode while guiding the paper through the arc which it must transverse through the bath is very difficult to achieve. As a practical matter, the result is often actual contact. The sliding contact between the image-bearing surface of the copy paper and the development electrode produces a smearing of toner particles across the image-bearing surface of the copy paper, visible as distinct black smear lines which are highly objectional. Any attempt to avoid smearing by widening the space between the copy paper and the development electrode reduces its effectiveness in eliminating edge effect.